OUTLINE: This is a randomized, double-blind, placebo-controlled, multicenter study. Patients are stratified according to premalignant risk (early vs advanced). Patients in each stratum are randomized to 1 of 3 treatments arms. Arm I: Patients receive lower-dose oral celecoxib twice daily. Arm II: Patients receive higher-dose oral celecoxib twice daily. Arm III: Patients receive oral placebo twice daily. Treatment continues in all 3 arms for 12 weeks in the absence of disease progression or unacceptable toxicity. Patients are followed at 18, 24, and 26 weeks.